What is the name of the first generation of offspring in Mendel's crosses?

The first generation of offspring in Mendel’s crosses is called the F1 generation, short for first filial generation. In a typical Mendelian experiment, the F1 offspring were produced by crossing the original parental, or P, generation.

Mendel often began with true-breeding pea plants that were homozygous for a trait. When he crossed plants with contrasting characteristics—for example, purple-flowered plants with white-flowered plants—the resulting F1 plants inherited one allele from each parent. They were therefore commonly heterozygous, even when all displayed the dominant phenotype.

The F1 generation was important because its appearance revealed patterns of dominance, but the next generation supplied further evidence. Mendel allowed F1 plants to self-fertilize, producing the F2 generation. Recessive traits that were hidden in the F1 generation reappeared among the F2 offspring.

A common mix-up is calling the first offspring the P generation. P means parental generation, the organisms used in the initial cross. F1 means their offspring, while F2 means the offspring produced when F1 organisms are crossed or self-fertilized.